The invention relates to a mold apparatus for production of information-carrying disk blanks of polymethacrylate plastic by means of injection molding or compression molding, to a method for production of the information-carrying disk blanks and to the blanks and DVDs produced therefrom.
German Patent 3613334 C2 describes an injection mold for injection molding of information-carrying plates, especially compact disks, of polycarbonate or polymethyl methacrylate, the mold having two halves, wherein one half has a polished insert of zirconium oxide or silicon nitride in the region of the mold cavity. Detrimental influences such as accretions of dirt or deposition of moisture are prevented by this feature.
German Patent Application 19625717 A1 describes a method for cutting off the sprue of an injection-molded article, especially of an information-carrying disk, wherein the sprue is cut off outside the apparatus by means of a laser. In this way the sprue can be removed in particularly dust-free manner. The position of the disk-shaped sprue can then be freely chosen. In the case of location in the central region, advantageously no burrs are produced at the disk surface. Furthermore, the precise cutting edge of the laser permits an increase of the gate gap between sprue and mold part.
European Patent Application 551869 A1 describes a method and an injection-molding machine for production of information-carrying disks of thermoplastic plastic such as polycarbonate. In this case the plastic is first injected into the injection mold and then a movable cylinder is forced into the center of the injection molding, so that the resulting sprue piece remains joined to the disk only by a thin rim and can then be cut out with sufficient ease.
U.S. Pat. No. 5,330,693 also describes a method and an injection-molding machine for production of information-carrying disks from thermoplastic plastic. In this case it is important that a sufficient gap ranging from 0.2 to 0.9 mm, preferably 0.75 to 0.85 mm be left between the sprue piece and the disk, to ensure that the mold will be filled throughout its entire extent.
German Patent Application 3839536 A1 describes an injection-molding device for information-storage disks, in which the ejector device contains a compressed-gas channel. Polycarbonate and polymethyl methacrylate are cited as injection-molding materials. The problem of possible mold contamination by particulate dust during use of polymethyl methacrylate is not mentioned. Particulars of the dimensions of the injection gap or of the cutting gap are not provided.
Information-carrying disks of plastic are now produced predominantly of polycarbonate in injection-molding or compression-molding methods. A general problem therein is the relatively high tendency toward birefringence, which can easily occur, especially with polycarbonate, if processing is not optimal, especially in the case of thin layer thicknesses. During readout of information from the information-carrying disksxe2x80x94such as compact disks (CD), minidisks (MD) or digital versatile disks (DVD)xe2x80x94such birefringence leads to undesired interference and thus in the worst case to scrap loss.
Polymethacrylate plastics, or in other words plastics made predominantly from methyl methacrylate, exhibit less tendency to development of birefringence, and so the use of polymethacrylate plastics instead of polycarbonate certainly represents an interesting alternative. In addition to a larger processing window, polymethacrylate possesses the further advantage, compared with polycarbonate, of greater replication accuracy and surface hardness. A disadvantage, however, is greater brittleness. This causes problems in cutting off the sprue parts, since more particulate dust is formed and can get into the apparatus or onto the information-carrying disks themselves. Such particles can contaminate the injection mold or female dies, or in the worst case even make them unusable. Particulate dust on the information-carrying disks can lead to problems during subsequent processing steps, especially during metallization of the information-carrying side with an aluminum layer. This particulate dust leads to increased scrap loss and to an aluminum surface that is not closed.
It was regarded as an object to provide, for injection molding or compression molding of information-carrying disk blanks of polymethacrylate plastic, an apparatus which makes it possible to obtain information-carrying disk blanks with high replication accuracy and, while the sprue piece is still in the apparatus, to shear it off to such an extent that it can be easily cut off after removal of the blank. A particular objective was to prevent particle formation due to tearing out at the cutting-off point during the cutting-off process.
The object was achieved by a
mold apparatus, suitable as a component of an injection-molding or compression-molding machine, for injection molding or compression molding of information-carrying disk blanks (11) of polymethacrylate plastic, the mold apparatus comprising two mold parts (1, 2), wherein one mold part (1) contains a female die (8) with the information data and together with the second mold part (2) forms a cavity for the one information-carrying disk blank, a sprue bushing (4) for a sprue piece (5) with a centrally positioned gate (6), and an impact cylinder (9) for shearing the sprue piece (5) in the apparatus and an ejector pin (10) are also provided, characterized in that
the injection gap (SP) between the gate (6) of the sprue piece (5) and the information-carrying disk blank (11) is smaller than 0.3 mm and the cutting gap (SN) between the rim of the impact cylinder (9) and the information-carrying disk blank (11) is smaller than 0.2 mm.
The invention also relates to a method for production of an information-carrying disk blank of polymethacrylate plastic, as well as to the new information-carrying blank and DVDs produced therefrom.